The present invention refers to admixture of a hydrophobic polymeric dispersion for providing a cement mortar which after curing has a considerably reduced water absorption and thus an improved permanence than cement mortar manufactured in the conventional way.
A fresh cement based binding agent mixture (cement mortar or concrete mass) consists of solid particles (aggregate), water and air. A cement-bound concrete substantially consists of about 1 volume part of cement, 2 parts of water and 6,5 parts of aggregate; in addition to this there will be 50 l air in about 1000 l of fresh concrete mass. The cement particles react with a part of the mixing water to a hydration product which forms a mineral adhesive, the water-cement paste.
The fresh concrete or mortar contains water in order to on one hand together with the cement form the chemically hardening binding agent and on the other hand give the mass the consistency and homogeneity that is required for the compaction. In most cases the water admixture is considerably larger than required for the chemical reactions even if the degree of hydration is complete.
The water that is not chemically bound, the so called free water, forms capillary pores, which are continuous at the beginning of the hydration process and at high water-cement ratios. The volume of the capillary pores is therefore influenced by the water-cement ratio and the degree of hydration. The water in the capillary pores causes shinkage and swelling of hardened concrete, mortar and cement paste at drying-out and moistening resp. The considerably smaller pores, the paste pores, are contained in the water-cement paste, which makes the principle part of the hydration products. The volume of these pores are only determined by the volume of the paste and is completely independent of the initial water-cement ratio. The paste pores are above all determining of the shrinkage.
Besides the said pores air pores are contained, which are formed at the mixing. They can be natural so called comprimation pores and intentionally added pores. The natural air pore content in concrete, which is obtained when no special measures are taken is 1.0-3.5% by volume. Intentionally created air pores can be provided in many different ways, as well mechanisms as principles are described in the literature.
During the curing a number of chemical reactions take place, at which i.a. water disappears through the capillary system. The cured concrete thus has a capillary system, which can take up water by the fact that the walls are hydrophilic. In certain cases this can be a drawback, e.g. by the fact that water in the capillaries freezes at low temperatures and causes disintegration of the concrete. This gives problems with late strength of the concrete especially in cold and moist environments and at marine applications.
It is previously known through SE-B-Nos. 7614518-4 (publ. No. 418.736), 7614519-2 (publ. No. 418.852) and 8100489-7 (publ. No. 420.595) to prepare a cement mortar with low density (1200-2000 kg/m.sup.2) by adding small amounts (0.2-5%) of polymeric particles. This depends on that polymeric particles added to the mortar stabilizes air pores. The polymeric particles used contains a hydrophobic component consisting of e.g. acrylic or methacrylic monomers, styrene or butadiene and a non-ionic tenside or a hydrophilic component consisting of an ethylenically unsaturated polymerizable compound.
In the British patent specification 1.159.377 there is described a cement mortar for joining structural elements. Manufacture of concrete is not mentioned. In the cement mortar there is added alkyl acrylates having an alkyl chain of up to 18 carbons, at which ethyl acrylate is preferred. The effect of this admixture is told to be an improvement of the adhesion of the mortar.